The present invention relates to a color picture tube device and, more particularly, to a color picture tube device with an electron gun assembly for generating three electron beams.
In a conventional in-line type color picture tube device, three electron beams, e.g., R, G and B beams are generated from an electron gun assembly received in a neck section of a tube envelope. These electron beams are so converged as to obtain an optimal raster size at a panel section as a screen of the tube envelope. The electron beams are deflected by a deflection magnetic field produced by a deflection yoke which is located around the neck and funnel sections of the tube envelope and which comprises saddle type coils for generating a horizontal deflection magnetic field and a taroidal coil mounted around an annular magnetic permeable core in a taroidal manner so as to generate a vertical deflection magnetic field. The screen is scanned with the deflected electron beams.
In a self convergence type color picture tube, a horizontal deflection magnetic field is formed in a pin cushion shape, and a vertical deflection magnetic field is formed in a barrel shape. The three electron beams are converged on the entire region of the substantially rectangular screen, thereby sufficiently minimizing convergence errors. Methods for minimizing convergence error to improve image quality is disclosed in Japanese Patent Publications Nos. 58-45135 and 51-44046. A magnetic field control element of a high permeability magnetic material is located at a proper position between the deflection yoke and the electron gun assembly to shunt or enhance the magnetic field leaked from the deflection yoke, thereby equalizing the raster size traced by the center electron beam with that of the side electron beams. Furthermore, in Japanese Patent Publication No. 58-7017, two types of magnetic shunt elements are located at different planes along the axis of the envelope to increase a margin for correcting coma along the horizontal and vertical axes, thereby setting the coma along the horizontal and vertical axes within predetermined values.
However, in a color picture tube using the conventional magnetic field control element, the following drawback is presented. Most conventional color picture tubes employ a self convergence system wherein R, G and B electron beams are converged on the display screen. According to this system, electron beam convergence is performed by utilizing aberration components of the deflection magnetic field itself. Therefore, the horizontal deflection magnetic field must have a pin cushion shape, and the vertical deflection magnetic field must have a barrel shape. In addition, the magnetic field control element located at the top of the electron gun assembly received in a neck acts on the magnetic field leaked from the deflection yoke so as to converge a center beam and side beams on the screen.
The convergence of the center beam and the side beams is greatly degraded at corners of the screen.
FIG. 1 shows a screen wherein the scanning lines of the center beam of the screen are not coincident with these of the side beams 5R and 5B at corners of the screen. Referring to FIG. 1, the solid lines represent the scanning lines of side beams, and the broken lines represent the scanning lines of center beam. The abovementioned magnetic control element is generally designed to align the beams at top and bottom center points a and right and left center points b.
As shown in FIG. 1, the scanning lines of the center beam are shifted, as compared with these of the side beams, depending on the distance from the V axis to the scanning position of the center beam along the horizontal axis, thereby increasing convergence errors at the corners of the screen and hence degradation of the image quality. This degradation is unacceptable in a high-resolution character display. In addition, when the screen size and the deflection angle are increased, the above-mentioned convergence errors are increased.
At an intermediate point al along the V axis, the scanning point of the center beam is deviated outside the side beams. In this manner, even at the central portion of the screen, convergence is degraded.
In a conventional magnetic shunt element having a shape and arrangement as shown in FIG. 8 of Japanese Patent Publication No. 58-7017, the first magnetic shunt element at the cathode side acts to increase deflection sensitivity of the center beam with respect to the vertical or V axis. However, the second magnetic shunt element decreases deflection sensitivity of the center beam. Even in a color picture tube having the arrangement described above, the scanning lines of the center beam are shifted from these of the corresponding side beams near the corners of the screen.